It has been proposed for rigid metal pipes to be lined with a polymeric liner in the form of a polymeric sheath pressed against the internal wall of the rigid pipe. Such a technique for protecting the rigid pipe is described for example in WO 00/77587 A1. That document also describes special structures of the liner, one of the structures allowing the removal of the gases originating from the fluid flowing in the rigid pipe and diffusing through the liner, so as to limit the corrosion of the flexible metal pipe.
In production flexible pipes, the fluid being transported is often polyphase and contains gases such as H2S, CO2, methane, which can diffuse through the pressure sheath. Because of this diffusion of gas, the metallic elements of the structure of the flexible pipes, such as the carcass, the pressure armor layer(s) and tensile armor layer(s) need to be able to resist H2S. These metal elements are therefore expensive to manufacture because the manufacturing involves special treatments such as quenching, tempering, patenting or other additional operations, although the mechanical properties remain mediocre.
In flexible pipes said to be of the smooth-bore type, that is to say ones which, as their innermost element, have an internal sealing sheath which is also known as the pressure sheath, the gas originating from the fluid being transported and diffusing through the internal sealing sheath increases the pressure in the annulus, and this may lead to the collapse of the pressure sheath. As a result, provision is made for this gas to be drained off so as to limit the pressure in the annulus, drainage being performed through and along the tensile armor layer(s) to a valve which opens to the outside of the flexible pipe. However, in most cases, the flexible pipes of this type have an anti-crushing sheath, generally polymeric, situated above the pressure armor layer(s) and it becomes impossible to drain the gas away through the tensile armor layer(s) because the gas is stopped by the anti-collapse or anti-crush sheath situated under said armor layer(s) Because of this difficulty with draining the gas away, flexible pipes known as smooth-bore pipes are not used for production and are reserved for water-injection lines.
In pipes said to be of the rough-bore type, in which the innermost element consists of a metal carcass, an anti-crush sheath may also be used, this being arranged above the pressure armor layer(s).
In order to solve this problem, FR-B-2 775 052 proposes positioning the anti-collapse sheath between two pairs of armor layer(s) so that the gas is drained away in the lower part of the armor layer(s) which lies immediately below the anti-collapse sheath. Under such conditions, it becomes possible to use different materials for making the two pairs of armor layer(s). The pair of armor layer(s) situated under the anti-collapse sheath will be manufactured using a steel not resistant to H2S, and which is therefore less expensive and has better mechanical properties, whereas the steel used to manufacture the pair of armor layers which is in contact with the gas needs to be able to resist H2S and is therefore relatively expensive with mediocre mechanical properties and a greater weight.
Application WO 99/66246 describes a flexible pipe in which a layer is formed above the pressure armor layer(s) to allow a gas present in the annulus or diffusing through the pressure sheath to flow longitudinally.
It has also been anticipated, although this has not been exploited on an industrial scale, for pressure armor to be produced using shaped wires the cross sections of which have lateral recesses to form a duct for draining the gas away. Wires in the shape of a Z, U, T, I, etc. may have such lateral recesses. As these shaped wires are already relatively expensive and heavy, they will also need to be able to resist H2S, not to mention the significant pressure drops that occur along the entire length of the pressure armor.
WO 01/33130 A1 describes a flexible pipe comprising several layers of different materials and one of which comprises drainage passages, it being possible for this drainage to take place between the pressure sheath and the pressure armor layer(s) or alternatively, along the armor layer(s).
However, the gas drained away is still in contact with the metal elements (pressure armor layer(s) or tensile armor layer(s)), which means that the materials employed have to be able to resist the presence of H2S in particular.
The solutions proposed in the prior art described. hereinabove are unable to eliminate the disadvantages associated with the presence of gas in the annulus and in particular are unable to solve the problems of corrosion.
In another technical field such as the extrusion of plastic tubes, WO 90/02648, for example, has proposed the forming of longitudinal channels in the thickness of the tube. The purpose of these channels is to lighten the plastic tubes rather than to allow any drainage of corrosive gases.